Composite laminate with self-healing layer

ABSTRACT

A composite structure comprising: a first stack comprising a plurality of plies of composite material and at least one ply of self-healing material, the ply of self-healing material comprising a plurality of containers each containing a curable healing liquid; and a second stack comprising a plurality of plies of composite material, the stacks being joined together at a bond line. By placing a ply of self-healing material in one of the stacks (preferably relatively close to the bond line) the ply of self-healing material can resist the propagation of cracks between the first stack and the second stack. Preferably the global strength of the first stack is greater than the global strength of the second stack.

FIELD OF THE INVENTION

The present invention relates to a composite laminate structure, and amethod of forming a joint between a pair of composite structures.

BACKGROUND OF THE INVENTION

FIG. 1 a shows a bonded joint between a primary structure 40 and asecondary structure 41. Due to fatigue loading, cracks will propagateand follow a path which cannot be previously determined. Three differentscenarios may be identified.

-   -   The crack 43 b propagates through the secondary structure 41,        parallel to the bond line 42 (FIG. 1 b). This will cause a        global failure of the secondary structure 41, which will stop        its collaboration with the primary structure 40. Generally this        scenario is not catastrophic. If a fail-safe design philosophy        is employed, failure of the secondary structure 41 will not        generate global failure of the structure, which remains capable        of withstanding the external loads.    -   The crack 43 c propagates toward the external surface of        secondary structure 41 and extinguishes (FIG. 1 c). This        scenario can either cause global failure of secondary structure        41 or partially reduce its capability. In both cases, this will        not cause global catastrophic failure of the whole structure, as        the primary structure 40 remains pristine.    -   The crack 43 d propagates toward the internal surface of        secondary structure 41 and then through the primary structure 40        (FIG. 1 d). This scenario is not acceptable since it leads to        catastrophic failure of the primary structure 40 and therefore        must be avoided.

The ability to confine the crack within an established perimeter wouldsimplify certification activity, increase the level of confidence,improve the reserve factor and the final global weight, and finally,increase safety.

Various self-healing structures are described in “Bioinspiredself-healing of advanced composite structures using hollow glassfibres”, R. S. Trask, G. J. Williams and I. P. Bond, J.R. Soc. Interface(2007) 4, 363-371, (doi:10.1098/rsif.2006.0194). A sixteen-plyglass-fibre laminate is described in which self-healing filaments wereintroduced at four damage critical ply interfaces. A sixteen-plycarbon-fibre laminate is also described in which healing glass fibre(HGF) was located at two interfaces within the lay-up, wound directlyonto uncured carbon-fibre reinforced plastic (CFRP) plies prior tolamination. According to Trask et al, the incorporation of HGF asdiscrete plies was deemed unsuitable for CFRP laminates as it wouldeffectively produce a hybrid glass-carbon laminate and result in asignificant reduction to their outstanding mechanical properties.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a composite structurecomprising: a first stack comprising a plurality of plies of compositematerial and at least one ply of self-healing material, the ply ofself-healing material comprising a plurality of containers eachcontaining a curable healing liquid; and a second stack comprising aplurality of plies of composite material, the stacks being joinedtogether at a bond line.

A further aspect of the invention provides a method of forming acomposite joint, the method comprising: assembling a first stackcomprising a plurality of plies of composite material and at least oneply of self-healing material, the ply of self-healing materialcomprising a plurality of containers each containing a curable healingliquid; assembling a second stack comprising a plurality of plies ofcomposite material; and bonding the stacks together at a bond line afterthey have both been assembled.

A further aspect of the invention provides a method of deflecting acrack in the composite structure of the first aspect of the invention,the crack originating in the bond line or the second stack, the methodcomprising breaking at least some of the containers in the self-healinglayer such that the curable healing liquid flows from the brokencontainers and cures, thereby deflecting the crack.

By placing a ply of self-healing material in one of the stacks(preferably relatively close to the bond line) the ply of self-healingmaterial can resist the propagation of cracks between the first stackand the second stack. Preferably the global strength of the first stackis greater than the global strength of the second stack.

Typically the first stack comprises N plies arranged in a stackingsequence including a first ply at one end of the stacking sequence whichis adjacent to the bond line and an Nth ply at another end of thestacking sequence which is remote from the bond line, and wherein theply of self-healing material is located relatively close to the bondline in the sense that its position in the stacking sequence is lessthan N/2.

Typically the ply of self-healing material is the first, second, thirdor fourth ply in the stacking sequence. Preferably the ply ofself-healing material is not the first ply in the stacking sequence.

The stacks may be bonded together by a layer of adhesive. In this casethe bond line will have a thickness equal to the thickness of the layerof adhesive. Alternatively the stacks may be bonded together byco-curing them. In this case the bond line will have zero thickness.

A further aspect of the invention provides a composite laminatestructure comprising: a stack of plies of composite material, each plyof composite material comprising a plurality of reinforcement fibresembedded within a matrix; and one or more plies of self-healing materialembedded within the stack of plies of composite material, each ply ofself-healing material comprising a plurality of containers eachcontaining a curable healing liquid, wherein the stack has a totalthickness T2 and the plies of self-healing material have a totalthickness T1, and wherein the ratio T1/T2 is less 0.1.

By making the ratio T1/T2 relatively low, then the propagation of crackscan be arrested without having a significantly deleterious effect on thebuckling performance of the structure.

The following comments apply to all aspects of the invention.

The containers may be formed from a glass material or any other suitablematerial.

The containers and the reinforcement fibres may be formed from differentmaterials. In contrast to the teaching of Trask et al, it has been foundthat forming the containers from a different material does not result ina significant reduction to the mechanical properties of the laminate,particularly if the ratio T1/T2 is sufficiently low.

The ply of self-healing material may comprise a “prepreg” ply—that is, aply in which the plurality of containers are impregnated with a matrixbefore the “prepreg” ply is assembled in the stack. Alternatively,instead of being assembled in the stack as a discrete “prepreg” ply, thecontainers may be fibres which are wound directly onto an uncured ply ofthe composite material prior to lamination, as described in Trask et al.

In the embodiments described below, the ply of self-healing material hascontainers but no reinforcement fibres. However, in an alternativeembodiment the ply of self-healing material may comprise a plurality ofcontainers intermingled with a plurality of reinforcement fibres.

The containers may comprise fibres, vesicles, or any other suitablehollow structure.

The containers in the ply of self-healing material may contain aone-part system of curable healing liquid. Alternatively the ply ofself-healing material may further comprises a plurality of containerseach containing a hardener liquid which cures the curable healing liquidon contact with the curable healing liquid. In a further alternative acatalyst or hardener may be contained within the matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIGS. 1 a-1 d show a conventional bonded joint;

FIG. 2 shows a bonded joint according to a first embodiment of theinvention;

FIG. 3 shows a single ply of CFRP;

FIG. 4 shows the primary structure being laid-up;

FIG. 5 shows a single ply of self-healing material;

FIG. 6 shows two of the HGFs in detail;

FIG. 7 shows the two stacks being brought together prior to co-curing;

FIG. 8 shows a crack being deflected;

FIG. 9 shows a bonded joint according to a second embodiment of theinvention;

FIG. 10 shows a longitudinal section through an adhesively bonded jointbetween a stringer and a panel;

FIG. 11 shows a transverse section through the panel showing a pluralityof stringers; and

FIG. 12 is a graph showing how varying the number of crack deflectorscan affect the mechanical properties of the structure.

DETAILED DESCRIPTION OF EMBODIMENT(S)

The composite laminate structure shown in FIG. 2 comprises a primarystructure 1 and a secondary structure 2 joined at a bond line 14. Eachstructure 1, 2 is formed by assembling a stack of plies of compositematerial. Each ply of composite material comprises a plurality ofreinforcement fibres embedded within a matrix. An exemplary ply 3 isshown in FIG. 3. In this case the ply 3 is a so-called “prepreg” with asingle layer of unidirectional carbon fibres 4 impregnated with apartially cured epoxy resin matrix 5. For ease of illustration thefibres 4 in the figures are all shown directed in an out of the page.However in practice the direction of the fibres can vary through thestack to give desired mechanical properties to the stack.

FIG. 4 shows how the primary structure 1 is formed. A stack of N pliesof composite material is laid onto a table 50 in a stacking sequenceincluding a first ply (i=1) at the bottom of the stacking sequence andan Nth ply (i=N) at the top.

The fourth ply 6 is a layer of self-healing material (referred tohereinafter as a “crack deflector”). The crack deflector 6 is shown indetail in FIGS. 5 and 6 and comprises a “prepreg” ply with a pluralityof hollow fibres 8, 9 impregnated with a partially cured epoxy resinmatrix 15 before the “prepreg” ply is assembled in the stack. As shownin FIG. 6, each fibre 8 comprises a hollow glass fibre 10 containing aliquid adhesive 11 such as Araldite® 2021/A resin, and each fibre 9comprises a hollow glass fibre 12 containing a liquid catalyst 13 suchas Araldite® 2021/B hardener. Since the adhesive and catalyst arecontained in different hollow fibres, this ensures a longer shelf life.

For ease of illustration the fibres 8,9 in the figures are all showndirected in an out of the page, in the same direction as the fibres 4.However in practice the fibres 8, 9 can be oriented in any direction.

After the stack shown in FIG. 4 has been assembled, it is partiallycured in an autoclave. The secondary structure 2 is assembled andpartially cured in a similar way. Next, the partially cured structures1,2 are brought together as shown in FIG. 7 and placed in an autoclavewhere they are fully cured. During this co-curing process the structures1, 2 become bonded together at the bond line 14.

Note that the stacking order of the primary structure 1 may be reversedfrom that shown in FIG. 4, that is with the crack deflector 6 being theN−3rd ply in the sequence, and the top (Nth) ply being adjacent to thebond line 14 in FIG. 2. In both cases the crack deflector 6 is locatedrelatively close to the bond line.

The global strength of the secondary structure 2 is smaller than theglobal strength of the primary structure 1. Therefore if a crackinitiates during cyclic loading of the joint, it will most likely belocated somewhere within the secondary structure 2.

When the crack deflector 6 is impacted and/or undergoes fatigue cycles,the glass fibres 10, 12 break and the liquid adhesive 10 and catalyst 13flows out, infiltrating the void created by the crack. This triggers twoeffects, which work synergistically to mitigate or arrest thepropagation of cracks:

-   -   Energy absorption due to the liquid phase dumping propagation        shock waves    -   Solidification, which will replace the material broken by the        crack propagation with the solid adhesive system thus        guaranteeing material continuity and a smoother load path and        mitigating local stress concentrations at crack tips.

FIG. 8 shows the physical behaviour which occurs locally when a crackfront originating in the bond line 14 or the secondary structure 2encounters the crack deflector 6.

As soon as the crack 16 breaks the fibres in the crack deflector 6 theliquid adhesive will start infiltrating the void and solidifying to forma region of cured adhesive 20 thereby deflecting the crack. Evidently,due to the fatigue loading, the crack front will continue propagatingand breaking new fibres, causing new adhesive to flow out andinfiltrating the voids.

The adhesive/catalyst self-healing system can be “tuned” such that theamount of cured volume per unit time is comparable to the volume of voidcreated by the crack propagation in the unit time. FIG. 8 shows thepresence of a micro-void 21, which will remain after the crack isdeflected as shown in FIG. 8. This is due to the fact that the viscosityof the liquid system does not allow total infiltration of the crackpath. Minimization of the size of the micro-void 21 is desirable and canbe ensured by appropriate selection of the curing speed and viscosity ofthe healing system. The size of the micro-void should preferably becomparable with the maximum micro-void statistically present within thecrack deflector 6 as a result of the manufacturing process used.

FIG. 9 shows an alternative joint where instead of being co-cured, theprimary structure 1 a is bonded to the secondary structure 2 a by alayer of adhesive 16 which in this case defines the bond line of thestructure. In this case, the structures 1 a, 2 a are fully cured beforethey are bonded together. Note that in this example the fourth layer inthe secondary structure 2 a also contains a crack deflector 6.

FIG. 10 illustrates a pair of embedded self-healing layers 30, 31 in astringer run-out on a composite laminate panel 34. The panel 34 carriesa number of stiffening elements (known as “stringers”) which run alongthe length of the panel. The stringers are T-shaped as shown in FIG. 11,with a foot bonded to the panel 34 and a blade 35 extendingperpendicular to the panel 34. The foot of each stringer extends beyondthe blade as shown in FIG. 10 to form a so-called stringer run-out 33.

The upper self-healing layer 30 is embedded within a stack of “prepreg”composite plies which form the stringer run-out 33. The stringer run-out33 is bonded to the composite panel 34 by a layer of adhesive 35. Thecomposite panel 34 contains the lower self-healing layer 31. These twolayers 30, 31 together form a “crack tunnel” confining a crack 32between them.

Note that in the examples given above only a single crack deflector 6 isrequired in each component 1, 2. Note however that further crackdeflectors may be integrated into the stack if required. FIG. 12 is agraph showing how varying the number of crack deflectors can affect themechanical properties of the structure.

The X axis in FIG. 12 shows the ratio between T1 and T2, where T1 is thetotal thickness of crack deflectors and T2 is the total thickness of thestructure. T1 and T2 are shown for example in FIG. 4. Where more thanone crack deflector is present in the structure then T1 is the sum ofthe thicknesses of all of the crack deflectors.

Curve 51 shows a buckling allowables curve. That is, curve 51 shows theload at which the primary component 1 will buckle in response to acompressive load applied parallel to the plies in the stack. It can beseen that this buckling load 51 is at a maximum where no crackdeflectors are present, and gradually decreases as T1/T2 increases.

Curve 52 is a fracture mechanics allowables curve. That is, curve 52shows the allowable load for crack initiation and subsequent propagationinto the primary structure. It can be seen that this load 52 is at aminimum where no crack deflectors are present, and gradually increasesas T1/T2 increases. The curve 52 increases in this way because thechance of a crack propagating into the primary structure diminishes.

The point where the curves cross defines a threshold ratio 53 abovewhich point the addition of further crack deflectors will globallydegrade the mechanical performance of the structure. The threshold 53will vary depending on the geometry of the structure, the thickness ofthe plies, the directions of the fibres in the various layers, and thematerials used for the various plies. However in general it is expectedthat the threshold will be no greater than 0.3 in most cases, and mostlikely below 0.2.

The inventor has realised that by making the ratio T1/T2 significantlyless than the threshold 53, the propagation of cracks can be arrestedwithout having a significantly deleterious effect on the bucklingperformance of the structure. If we consider the stack of N plies shownin FIG. 4, then only 1/N of the plies are crack deflectors, where N istypically between 20 and 40. If we assume that the crack deflector 6 hasa thickness which is the same as the thickness of one of the plies ofprepreg 3, then this translates to a ratio T1/T2 which is less than orequal to 0.05.

Note that in the examples given above, the crack deflector is not thefirst ply in the stacking sequence immediately adjacent to the bondline. This is preferred for the following reason. To maximise theefficiency of the crack deflector it is preferred for the crack toinitiate outside the crack deflector itself. Therefore if a crackinitiates at the bond line (which is an area where a crack is likely toinitiate) then by placing a few plies of prepreg 3 between the bond lineand the crack deflector it is ensured that the crack will have to gothrough the full thickness of the crack deflector in order to propagateinto the primary structure.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

1. A composite structure comprising: a first stack comprising aplurality of plies of composite material and at least one ply ofself-healing material, the ply of self-healing material comprising aplurality of containers each containing a curable healing liquid; and asecond stack comprising a plurality of plies of composite material, thestacks being joined together at a bond line.
 2. The structure of claim 1wherein the first stack comprises N plies arranged in a stackingsequence including a first ply at one end of the stacking sequence whichis adjacent to the bond line and an Nth ply at another end of thestacking sequence which is remote from the bond line, and wherein theply of self-healing material is located relatively close to the bondline in the sense that its position in the stacking sequence is lessthan N/2.
 3. The structure of claim 2 wherein the ply of self-healingmaterial is the first, second, third or fourth ply in the stackingsequence.
 4. The structure of claim 2 wherein the ply of self-healingmaterial is not the first ply in the stacking sequence.
 5. The structureof claim 1 wherein the bond line comprises a layer of adhesive.
 6. Thestructure of claim 1 wherein the stacks are co-cured at the bond line.7. The structure of claim 1 wherein the first stack has a totalthickness T2 and the plies of self-healing material have a totalthickness T1, and wherein the ratio T1/T2 is less than 0.1. 8-13.(canceled)
 14. A method of forming a composite joint, the methodcomprising: assembling a first stack comprising a plurality of plies ofcomposite material and at least one ply of self-healing material, theply of self-healing material comprising a plurality of containers eachcontaining a curable healing liquid; assembling a second stackcomprising a plurality of plies of composite material; and bonding thestacks together at a bond line after they have both been assembled. 15.A method of deflecting a crack in the composite structure of claim 1,the crack originating in the bond line or the second stack, the methodcomprising breaking at least some of the containers in the self-healinglayer such that the curable healing liquid flows from the brokencontainers and cures, thereby deflecting the crack.
 16. A methodaccording to claim 14 wherein the first stack comprises N plies arrangedin a stacking sequence including a first ply at one end of the stackingsequence which is adjacent to the bond line and an Nth ply at anotherend of the stacking sequence which is remote from the bond line, andwherein the ply of self-healing material is located relatively close tothe bond line in the sense that its position in the stacking sequence isless than N/2.
 17. A method according to claim 16 wherein the ply ofself-healing material is a first, second, third or fourth ply in thestacking sequence.
 18. A method according to claim 16 wherein the ply ofself-healing material is not the first ply in the stacking sequence. 19.A method according to claim 14 wherein the bond line comprises a layerof adhesive.
 20. A method according to claim 14 wherein the stacks areco-cured at the bond line.
 21. A method according to claim 14 whereinthe first stack forms a primary structure, the second stack forms asecondary structure, and a global strength of the secondary structure isless than a global strength of the primary structure.
 22. A methodaccording to claim 14 wherein each ply of composite material in thefirst stack comprises a plurality of reinforcement fibres embeddedwithin a matrix, and each ply of composite material in the second stackcomprises a plurality of reinforcement fibres embedded within a matrix.